Hi!
I’m currently developing a flight simulator type game, and at the moment I’m working on the code for the actual physics of the aircraft, such as throttle and lift etc.
I’ve used AddRelativeForce to create my throttle and yeah, it works, but it just jumps to that speed.
So it’s not very realistic, therefore what I would like to happen is that the user must hold down the throttle until it reaches the correct speed, therefore making the speed increase over time (eg. at 5 seconds it’s at 50 m/s and after 20 seconds of holding it down it’s at 250 m/s).
This is probably very easy, but I’m new to coding so I haven’t seen it 
The code I’m using is:
if(Input.GetButtonDown ("Throttle"))
rigidbody.AddRelativeForce(0,0,-500000);
Thanks 
Rigidbody.AddRelativeForce should be adding force over time, and giving you that smooth acceleration that you’re looking for. However, I notice you have it set incredibly high, so it may simply be that you’re adding so much force that it looks like its starting instantly.
Try lowering the force value from -500000. Also, if that doesn’t work, there is a variable called rigidbody.drag. It slows down the speed of the rigidbody as it moves. You could try increasing the drag of your plane by setting the drag to be higher.
One last thing to look at is the mass. AddForce and AddRelativeForce both take the mass into account when looking at adding forces. Maybe you have your mass set too low, and the force just moves it too fast.
Fiddling with those three things should fix your problem.